Trihydroxy- and alkoxy-hydroxybenzenes are important structural components for the synthesis of pharmaceutical and cosmetic products, as well as for agro- and photo-chemicals (Kirk Othmer, Encyclopedia Chem. Technol., 3d edition, Vol. 18, page 670-84, 1982).
Until this time these compounds could be produced only at great cost and in several process steps.
For example, in European Patent No. 25 659, there is described a method of producing pyrogallol and substituted pyrogallol compounds by first of all transforming into suitable chlorine compounds by chlorination of the 2,6-dimethyl phenol derivative starting materials, and thereafter converting the chloromethylene groups thereof into aldehydes. The aldehydes were then oxidized in an alkaline medium into the corresponding pyrogallol and substituted pyrogallol compounds in accordance with the Dakin reaction, with peroxy compounds, such as hydrogen peroxide or peroxy acides, or their salts. However, this multistep process led to only moderate yields despite the considerable outlay.
Another process involving many steps is a modified Dakin reaction, which was carried out over the corresponding hydroxybenzaldehyde, and which led to satisfactory yields only if the pH level during the reaction was held continuously at 6-7 by addition of bases. Then the work up was also difficult in this process (European Patent No. 44 260).
Another complicated process with very variable yields of pyrogallol or pyrogallol compounds is disclosed in German OLS No. 26 53 446. A 2,2,6,6-tetrahalogen cyclohexanon must be produced first, which is then hydrolyzed with water in the presence of a catalyst or in the form of an alkoxide with acids.
Additional attempts using diaminophenols to obtain polyhydroxyl compounds did not lead to good results and were quite costly (German OLS No. 24 43 336).
Another possibility of obtaining hydroxy hydroquinone or substituted polyhydroxy compounds by means of the Thiele-Winter reaction was both very complicated and technically costly. The reaction actually consists of the conversion of a suitable quinone into a hydroxy-hydroquinone by acetoxylation with acetic acid anhydride in the presence of sulfuric acid or boron trifluoride etherate to the corresponding triacetate and then its conversion into the desired hydroxy hydroquinone or into substituted polyhydroxy compounds (German OLS No. 24 59 059).
Until this time, the preparation of 2,6-dimethoxy phenol occurred as a result of complete methylation of gallic acid, subsequent decarboxylation and selective demethylation of the 2-methoxy function. The 2,6-dimethoxyphenol yield was low, since only a 50% yield could be obtained in the first step of the decarboxylation (J. Am. Chem. Soc. 72, 1950, page 4986). Another process is described in the literature to be found in J. Org. Chem. 1984, 49, 4740 and J. Chem. Soc. Perkin Trans. I 1974, page 1353.
With the heretofore known process aromatic aldehyde compounds are reacted through a Baeyer-Villiger oxidation into the desired phenols with hydrogen peroxide or peroxy acids. Although fairly good yields can be obtained with this process, the disadvantage has been noted of a very long reaction time, actually up to 14 or 16 hours. In addition, this process also requires many steps.